1. Field of the Invention
This invention relates to an oven liner for use in a dielectric oven for heating a product, such as for cooking foodstuffs. Particularly, it relates to an oven liner that fits within a heating cavity of the dielectric oven and prevents arcing and flashover during operation of the oven. Further, such oven liners may direct cooking vapors, such as steam or vaporized oils or fats, out of the oven to improve heating efficiency.
2. Description of the Related Art
Commercial ovens are commonly convection ovens utilizing a slow convection heating process. Dielectric ovens, however, heat a product due to the electric, i.e., dielectric, losses caused when the product is placed in a varying electromagnetic field. If the product is homogeneous and the electromagnetic field is uniform, heat may develop uniformly and simultaneously throughout the mass of the product.
Ovens utilizing dielectric high frequency heating are known, and examples of such ovens are disclosed in U.S. Pat. Nos. 4,812,609 to Butot; 4,978,826 to DeRuiter et al.; and 4,980,530 to Butot, which are incorporated herein by reference. Such ovens may operate in a frequency range of about 2 to 40 Mhz. Further, dielectric ovens may be fitted with guide racks for stacking a plurality of trays carrying the product to be heated within an electromagnetic field or fields.
Dielectric ovens may utilize an oscillating circuit or circuits utilizing specially designed power sources, e.g., power tubes. Such oscillating circuits generally provide a substantially fixed distribution of voltage and power within a heating cavity. Thus, longer heating times may be required for heating greater volumes of products. Further, frequencies at which the ovens are operated are dependent on the characteristics of the product being heated.
A dielectric oven may include a heating cavity for receiving a tray containing the product during heating, a high frequency oscillating circuit, a power source for generating a high frequency electric signal, and electrodes for producing an electromagnetic field in the cavity to transfer power from the oscillating circuit to the product. The oven may be broadly operable for increasing the power transferred from the oscillating circuit to the product without increasing the operating voltage of the power source or the frequency of its operation. Further, such ovens may include a plurality of oscillating circuits having substantially similar resonant frequencies.
Each of the oscillating circuits may also include an inductance and a capacitance. The capacitance may include a pair of capacitors respectively formed between two electrodes of the oscillating circuit and additional plates or, for example, wall portions of the heating cavity. Preferably, the two electrodes of each oscillating circuit are oriented to produce an open electromagnetic field therebetween. In this configuration, electrode pairs form a pair of interconnecting load capacitors between the electrodes of the oscillating circuits. The dielectric of the load capacitors includes the product placed between the electrodes of each capacitor. The open electromagnetic field has a power intensity distribution determined by the dielectric characteristic of the product, while permitting the power source to operate at a substantially constant power level. Further, the use of the load capacitors as connectors between oscillators isolates the frequency of oscillation of the oscillating circuits from the effects of the dielectric characteristics of the product.
The product may be bracketed by the plates of a capacitor, i.e., the electrodes, in the oscillating circuit. The oscillating circuit may be arranged to provide a voltage across the capacitor which is twice the voltage across the power source, thus, permitting the doubling of distance between the electrodes without reducing the electromagnetic field strength. This allows the quantities of the product which may be heated between the electrodes to be increased.
When a product is heated in a dielectric oven or when the heating fluid surrounding the product is heated, e.g., boiled, to heat the product, vapors are produced in the heating cavity. Referring to FIG. 1, dielectric oven designs may include a housing 200 defining a heating cavity 202. Such oven designs have an access (not shown) through which trays 203 may be inserted and removed from the oven. A control unit 204 regulates the delivery of current from an electromagnetic energy source, such as power tube 205, to electrodes 206a and 206b. An insulated wall 207 of cavity 202 forms the dielectric between capacitors formed by electrodes 206a and 206b and grounded portions of housing 200.
Arcing or flashover may occur if the vapors are allowed to reach the capacitor plates, e.g., the oven housing 200. Arcing or arc discharge is a direct electrical current between two electrodes in a vapor, having a high current density and a relatively low voltage drop. Flashover is an electrical discharge around or over the surface of an insulator. Further, if vapors are allowed to pass from cavity 202 into the portion of the oven containing the tube and the control unit, the oven may be seriously damaged.
The presence of vapors, such as steam, in dielectric ovens is undesirable for other reasons. Steam is highly corrosive and may damage metal surfaces in the oven. Moreover, vapors may condense on the capacitor plates, e.g., electrodes, and foul their surfaces. Such vapors also may cause localized pitting or general oxidation which further reduces efficiency. The production of vapors within the electromagnetic field also may waste energy due to dielectric losses in the vapor cloud.